Inorganic polymers with a backbone of alternating phosphorus and nitrogen atoms are known as polyphosphazenes and have the general structural formula [(OR).sub.2 P=N].sub.n, where n is between 20 and 30,000, preferably 100 and 20,000. Synthesis for such polymers, with a variety of substituents at the phosphorus, are known. Allcock, H. R., Phosphorus-Nitrogen Compounds, Academic Press, New York (1972). Such polyphosphazenes exhibit and impart useful properties such as fire retardancy, low temperature flexibility, resistance to chemical attack, biocompatability and thermotropic or liquid crystalline behavior.
The most commonly used synthetic route to linear polyphosphazene is a two step process developed in the mid-1960's and disclosed by Allcock, H. R. et al. in Polymer Preprints, 21:111 (1980), the disclosure of which is incorporated herein by reference. The first step of the process is the ring-opening polymerization of the cyclic trimer, hexachlorocyclotriphosphazene, to produce linear high molecular weight polydichlorophosphazene. The ring-opening of hexachlorocyclotriphosphazene to produce polydichlorophosphazene is accomplished by either melt polymerization or solution polymerization as described by Konecny, J. O. et al. in J. Polymer Sci., 36:195 (1959), Jacques, J. K. et al. in J. Chem. Soc. London, 2112 (1965) and Allcock et al. in Inorganic Chem., 5:1709 (1966), the disclosures of which are all incorporated herein by reference.
Melt polymerization of hexachlorocyclotriphosphazene is usually carried out in evacuated sealed vessels at temperatures between 230.degree.-300.degree. C. in which heating is continued until the reaction mixture ceases to flow. If polymerization is continued after this stage, the polydichlorophosphazene may crosslink and become unsuitable for further substitution. Crosslinking is very unpredictable and is generally enhanced by the presence of impurities. The reaction time needed to transform hexachlorocyclotriphosphazene into polydichlorophosphazene by melt polymerization synthesis techniques is approximately 24 to 72 hours. When melt polymerization is used in the synthesis of polydichlorophosphazene, the molecular weight distribution of the polyphosphazenes may be less than optimal due to unpredictable crosslinking branching resulting from the long reaction time needed to transform hexachlorocyclotriphosphazene into polydichlorophosphazene.
Solution polymerization of hexachlorocyclotriphosphazene is known involving reacting, in a container attached to a condenser, hexachlorocyclotriphosphazene, 1,2,4-trichlorobenzene and a catalyst. Dry nitrogen is then bubbled through the reaction mixture. The reaction mixture temperature is maintained at approximately 210.degree. C. The reaction time necessary for the hexachlorocyclotriphosphazene to transform into polydichlorophosphazene by solution polymerization is typically less than 3 hours. In solution polymerization the viscosity of the reaction mixture is relatively low and the chain structure can be better controlled. Solution polymerization produces higher molecular weight polymers than does melt polymerization as a result of less branching due to a shorter reaction time. Solution polymerization is generally preferred to melt polymerization.
U.S. Pat. No. 4,242,316 discloses a synthesis for polyphosphazene by solution polymerization in which sulfamic acid is used as a catalyst. However, we have found that polymerization does not occur when sulfamic acid is used in its pure or undecomposed state, but rather occurs only when sulfamic acid is used in its decomposed form.
The second step in the synthesis of linear polyphosphazene is the nucleophilic substitution of the chlorine groups attached to the polydichlorophosphazenes with organic groups. Such nucleophilic substitution is necessary because of the facile hydrolysis of the chlorine substituents of the polydichlorophosphazene in the presence of atmospheric moisture. Such substitution is generally accomplished by reacting polydichlorophosphazene with an alkoxy or aryloxy sodium salt in a suitable solvent, such as tetrahydrofuran, chlorobenzene, benzene or toluene.
An improvement in the speed of the solution ring-opening step of the polymerization process of hexachlorotriphosphazene would be advantageous because it would provide for a higher degree of selectivity in the molecular weight distribution of the polyphosphazene due to the ability of the nucleophilic substituent of the alkoxy or aryloxy salt in the second step to react with the relatively narrow range molecular weight polydichlorophosphazene produced in the first step.